I. Field of the Invention
This invention relates generally to implantable body organ stimulators, and more specifically to a demand-type cardiac pacer whose sensing circuitry is capable of working with either bipolar or unipolar style pacing/sensing leads.
II. Discussion of the Prior Art
Since the late 1950's and early 1960's, considerable progress has been made in the design and manufacture of cardiac pacers. Early models were asynchronous in operation, producing stimulating pulses at regularly spaced intervals over the life of the power supply (batteries) used therein. These pacemakers comprise an electronic pulse generating circuit which is suitably encapsulated so that the device can be totally implanted within the body of the recipient and the stimulating pulses emanating from the pulse generator are coupled to the organ (heart) to be stimulated by way of elongated flexible pacing leads. Basically, two forms of pacing leads have been used--unipolar and bipolar. A unipolar lead commonly comprises an elongated flexible conductor embedded in an insulating body and extending from a distal tip electrode to a male terminal pin located at the proximal end of the lead. This terminal pin is adapted to mate with an appropriate female output jack on the pacemaker. The indifferent electrode for the unipolar pacing system generally comprises a conductive portion on the housing for the pacemaker electronics which is large when compared to the area of the tip electrode. A bipolar lead, on the other hand, generally includes a pair of elongated, flexible, conductive filaments embedded in a suitable insulative material, one of the conductors being connected at its distal end to a tip electrode and the other conductor being connected to a ring or spot electrode disposed on the surface of the insulative body and located a short predetermined distance proximally of the tip electrode. In the bipolar configuration, the proximal ends of the elongated conductors are connected to electrically discrete areas on a male terminal pin which is adapted to mate with the female jack on the pacer itself. A pacer-generated pulse would be applied between the tip and ring electrodes.
Following the development of the asynchronous pacer, so-called demand-type pacers were invented and functioned to inhibit artificial stimulation of the heart in the event that the heart was beating of its own accord. In a demand-type pacer, a sensing means is provided for detecting the occurrence of natural R-wave activity in the body and for resetting a resettable timing means upon the occurrence of such a R-wave. The pulse generator then produces a stimulating output signal only if the resettable timing means is not reset within a prescribed predetermined interval.
Demand-type pacemakers of the type described are also classified as either unipolar or bipolar, depending upon the manner in which pacing and sensing is accomplished. If the sensed R-wave signals are developed between the tip electrode and the metallic body of the pacemaker, sensing is said to be unipolar. Similarly, if the detected R-wave activity is developed between the tip electrode and the proximally located ring electrode, sensing is said to be bipolar.
In that pacemakers can use one of two types of leads, the possibility exists for an implanting physician to mistakenly join a unipolar lead to a pacer which is specifically designed for bipolar operation and vice-versa. This could lead to the implantation of a non-functioning system if the error should go undetected. Perhaps of a greater likelihood is the possibility that one may make a mistake when the pacer is designed to be programmable between the types of leads. The physician may use an external programmer to call for a bipolar lead when, in fact, a unipolar lead has been implanted and vice-versa.
The Bowers et al U.S. Pat. No. 3,735,766 is directed to an early attempt to provide an asynchronous pacemaker capable of operating in either a unipolar or a bipolar mode. The system of the Bowers patent contemplates that when the pacemaker is sold, it will be fully encapsulated within an insulative coating. To achieve bipolar pacing, a two conductor lead would be coupled between the pacer's connector block and the heart. Bipolar pacing will then take place between the two electrode elements disposed near the distal end of the catheter. To achieve unipolar pacing with the device of the Bowers et al patent, a portion of the insulative coating is capable of being readily peeled away from the pacer body, exposing an indifferent electrode area. The indifferent electrode is connected to one side of the pacer's pulse generator while the connector pin of a single-conductor (unipolar) lead mates with the output connection of the pulse generator. The Bowers et al patent is strictly an asynchronous pacer and is incapable of operating in a demand mode. The pacer system is not self-adaptive to the particular type of lead employed and the physician must be sure to join a bipolar lead to the pacemaker when bipolar pacing is to take place and a unipolar lead when unipolar pacing is to take place. In addition, he must remove the insulative covering from the pacer body to expose the indifferent electrode when unipolar pacing is desired.
The Bernstein U.S. Pat. No. 4,289,134 describes a catheter arrangement including built-in electronic circuitry so that when this special-purpose lead is used with a bipolar demand-type cardiac pacer, it converts a bipolar pacing system into a tripolar pacing system in which stimulation is applied between two intra-ventricular electrodes, while sensing of ventricular depolarization is obtained by an alternative pair of electrodes. Thus, when the Bernstein lead arrangement is available, a physician has the option of sensing in either a bipolar fashion between two intra-ventricular electrodes by using a standard bipolar lead, or in a unipolar fashion between one electrode located in the ventricle and the return electrode in the atrium by using the Bernstein lead itself.
The Peers-Trevarton U.S. Pat. No. 4,301,805 describes a connector system usable with either a unipolar lead or a bipolar lead. Whether the pacemaker operates in a bipolar or unipolar fashion depends again upon the particular mechanical connections made by the physician.
Lastly, the Duggan U.S. Pat. No. 4,402,322 describes a pacer system which can function in either a bipolar or a unipolar mode. In the system of the Duggan patent, however, the mode is either fixed during the manufacturing process or, alternatively, may be alterable through programming of the pacer via a telemetry mechanism.